A portable communication device, such as a cell phone and a smart phone, has spread. The duration time of a battery is considered as an important performance of the portable communication device. One of the parameters which influence the duration time of the battery is an amplification efficiency of a power amplifier which amplifies an electric power of a signal transmitted from an antenna.
As an output power becomes large, the power amplifier indicates a high amplification efficiency. In the range in which the output power becomes maximum (e.g. 24 (dBm)), the amplification efficiency is highest. Considering the actual use status of a portable communication device, the use frequency in the maximum output power is low. Since the use frequency becomes highest in the range of approximately the half of the maximum output power or less than it, high amplification efficiency is not acquired. Here, the output power is determined according to a distance to a base station, a type of communication (data, a sound, and so on), and a communication method (GSM (Global System for Mobile Communications, registered trademark) and so on), for example.
In order to realize high efficiency, the power amplifier called a multi-mode power amplifier is used. With the multi-mode power amplifier, the highest amplification efficiencies are acquired in different output powers, respectively, by using a plurality of amplifying elements properly having different sizes. As the sizes of the amplifying elements become large, the output powers when the amplification efficiencies becomes highest are large. When the output powers are a small level, a middle level and a large level (e.g. 24 (dBm), 14 (dBm) and 4 (dBm)) in order of the large size, three kinds of amplifying elements whose amplification efficiencies become highest are used properly according to the output powers, and hence high amplification efficiencies are acquired.
With respect to the multi-mode power amplifier, Patent Document 1 (i.e., U.S. Pat. No. 7,728,661) discloses a configuration in which amplifiers with high output power and amplifiers with low output power which amplify a RF (Radio Frequency) signal are coupled to a load resistance through transformers, respectively.